There are some parts Id like to make a few copies of (tracks,tarps, bags, helmets etc.)
I got myself some blue stuff to make a mold but wondering what would be the best resin for casting the parts.
I tried using 2 part Miliput but it seems too brittle.
Any alternatives?

Milliput is okay for bags , crates etc, but you really need to get a Polyurethane resin for thin parts like track ends, etc. If you are going to get resin, then just do all your casting with it as shelf life can vary..

I use this
https://www.barnes.com.au/rigid/easycast-1306
in Australia, but see if you can find something like it locally.

Perfect thank you!

I have recently used green stuff's acrylic resin and have been very happy with it:
http://www.greenstuffworld.com/en/mold-making/718-acrylic-resin.html?search_query=resin&results=77

Comes in powder and mix with water, no nasty smells or dirty containers to clean and I bet it may last a long time if well stored.
On the other hand, the material resulting is a bit like plaster (but very fine), unlike traditional resin.

Always had great success with Alumilite.